The Diurnal Cycle in Circulation and Cloudiness over the Subtropical Southeast Pacific: A Modeling Study

The extensive and persistent deck of stratocumulus (Sc) over the subtropical southeast Pacific (SSEP) plays an important role in the regional and global climate. As in other subtropical regions, the Sc form at the top of a marine boundary layer (MBL), capped by the subsidence inversion. A distinctive feature of this subtropical deck is its pronounced dawn-to-afternoon decrease in cloud amount and liquid water path, partially associated with a regular and marked descent of the inversion base and the warming of the lower troposphere. Furthermore, coastal observations in this area reveal a diurnal cycle in air temperature encompassing up to 5 km MSL. In this work, 15-day regional numerical simulations using the fifth-generation PSU–NCAR Mesoscale Model (MM5) in November (austral spring), May (late fall), and January (summer) 2001 were used to document the mean diurnal cycle in circulation and low-level cloudiness over the SSEP. The simulated amplitude, depth, and phase of the diurnal cycle in air temperature, wind, and cloudiness at the northern coast of Chile and over open ocean compare quite favorably with their observational counterparts. Large-scale subsidence prevails over the SSEP on a daily average. Between 1 and 5 km, however, the vertical velocity exhibits a marked diurnal cycle, largely produced by a band of upward motion propagating from the southern coast of Peru into the SSEP during late afternoon and night. Such an “upsidence wave” was found in the three simulations. The upsidence wave produces a significant cooling, leading to a consistent diurnal cycle in air temperature in low- and midlevels over the SSEP. The impact of the vertical velocity cycle on the MBL was further studied using a 1D version of the MM5 with higher resolution. The deepening of the MBL during the upsidence period induces a more turbulent MBL and more entrainment. The warming and drying of the MBL result in a greater dissipation of the cloud layer in the afternoon, increasing the amplitude of the diurnal cycle in Sc cloud amount with respect to the cycle forced by absorption of solar radiation only.